1. Field of the Invention
This invention relates to electronic parts and to their production using negative working radiation sensitive compositions.
2. Background Information
We have developed novel radiation sensitive compositions and in our earlier-filed U.S. Pat. Nos. 5,372,907 and 5,919,601 we disclosed these novel compositions and their use as imagable coatings for lithographic printing form precursors. We later determined that such compositions are suitable as radiation sensitive coatings for printed circuit and other electronic part precursors. However they are not optimal for use as coatings on all electronic part precursors. In particular, they are not optimal for use as coatings on thin, flexible substrates which are the precursors for some electronic parts and they are not flexible substrates which are the precursors for some electronic parts and they are not optimal for use on substrates required to be coated and imaged on both sides. For such applications we have sought a composition which can better withstand flexure and surface contact, without becoming scratched, cracked, flaked or delaminated. We have now devised a particular class of heat sensitive compositions which are extremely suitable for such applications.
The coatings used in pattern forming methods for electronic parts such as printed circuits are classified into two types: negative working and positive working. After exposure to radiation and development, the coating pattern is used as a screen for forming the patterns onto the underlying electronic elementsxe2x80x94for example by etching an underlying copper foil. Due to the high resolution demands and the requirements of high resistance to etching techniques, positive working systems are widely used. In particular, in the main there have been used alkali developable positive working coatings mainly composed of alkali-soluble novolac resins as disclosed in J.C. Streiter, Kodak Microelectronics Seminar Proceedings, 1979, p. 116. The primary active component of such positive working compositions, both in the context of lithographic printing forms and electronic parts, is a naphthoquinonediazide (NQD) derivative.
The types of electronic parts whose manufacture may use a radiation sensitive coating include printed wiring boards (PWBs), thick- and thin-film circuits, comprising passive elements such as resistors, capacitors and inductors; multichip devices (MDCs); integrated circuits (ICs); and active semiconductor devices. The electronic parts may suitably comprise conductors, for example: copper board; semiconductors, for example silicon or germanium; and insulators, for example silica as a surface layer with silicon beneath, with the silica being selectively etched away to expose portions of the silicon beneath (a step in the manufacture of, e.g., field effect transistors).
In U.S. Pat. No. 5,372,907 there is described a radiation-sensitive composition for a lithographic printing plate. The composition can be used in positive-working and negative-working manners. The composition comprises a resole resin, a novolac resin, a latent Bronsted acid and an infra-red absorber. The negative-working method involves imagewise exposure to activating radiation, heating overall to effect cross-linking selectively in those regions which had been imagewise exposed, and development to selectively remove the other regions. This composition and method gives excellent imaging for printing plates, and is in commercial use. However, it is unsuitable for use in the field of electronic part manufacture because PCB substrates would not withstand flexure and surface contact without becoming scratched, cracked, flaked or delaminated.
U.S. Pat. No. 5,919,601 describes similar technology, involving use of a composition comprising a binder resin, a cross-linking resin and a thermally activated acid generator as a printing plate coating. The plates were imaged using IR radiation, then heated overall to produce the required cross-linking reactions, then developed, to remove the regions which had not received the IR radiation. Again, imaging properties are believed to be good but mechanical properties are believed to be inadequate for applications involving the manufacture of electronic parts, such as PCBs.
U.S. Pat. No. 5,527,659 describes a chemical amplification resist composition comprising a binder, a photochemical acid generator, and a squarylium compound. Example 1 describes its coating onto a grained and anodised plate. The binder was poly-p-hydroxystyrene. It was subjected to radiation of wavelength of 630 nm, then heat treated, then developed, then inked up, for printing. Other binders are also described.
EP 819985A describes negative-working lithographic printing plates having a phenolic hydroxy group-containing resin, a latent Bronsted acid, an infra-red absorber and an amino cross-linking agent. The plates are subjected imagewise to infra-red radiation, then subjected to post-exposure bake, prior to development to remove non-imaged areas. Suitable resins are said to include novolacs and polyvinylphenols. EP 819980A is a very similar disclosure, where the infra-red absorber is carbon black. Suitable resins are stated to include novolacs, polyvinylphenols and phenolic hydroxy functionalized derivatives of poly(meth)acrylates, which can be synthesized starting from, for example, hydroxyethyl(meth)acrylate.
U.S. Pat. No. 5,814,431 describes a negative-working photosensitive resin used for a lithographic printing plate, and comprising a novolac or polyvinyl phenol resin, an amino compound able to cure the composition, a defined cyanine or polymethine compound able to absorb near infra-red radiation, and a photosensitive acid-forming generator. When a polyvinyl phenol resin is used an unsubstituted polyvinyl phenol resin is preferred, but hydroxyl groups thereof are protected by t-butoxycarbonyl, pyranyl or furanyl groups. Imaging involves laser irradiation, typically at 830 nm, a post-exposure bake, and development, to remove the regions which had not been struck by the laser beam. The plates are said to have good stability, good sensitivity and good inking-up properties.
U.S. Pat. No. 5,725,994 discloses novel negative-working photosensitive compositions useful for making heat mode lithographic printing plates. The compositions comprise an acid precursor, a particular hydroxyimide compound, and a hydroxyl group-containing linear polymer. The linear polymer may be an acrylate polymer. Imaging involves delivery of light, for example from a mercury lamp, then the heating of the imaged part, prior to development.
EP-A-899614 discloses a negative-working image recording material having excellent storability, and suitable for printing plate production. The material includes a specified diazonium salt, an infra-red absorber, and acid-triggered cross-linking agent and a binder. The binder may be selected from a wide range of polymers, including novolacs and acrylates. Imaging involves imagewise delivery of infra-red radiation followed by a post-exposure bake of short duration, prior to development.
EP-A-874282 describes negative-working image recording materials comprising a heat-triggered acid generator, an acid-triggered cross-linking agent, a binder polymer and an infra-red absorber. The materials are said to be for a lithographic printing plate, a color proof, a photoresist or a color filter, and are claimed to have a high sensitivity. In the general description a very wide range of binder polymers is described. In the examples, which are all of lithographic printing plates, the binder polymers employed are phenol resins, styrenic resins and methacrylate resins, of various structures.
WO 97/39894 describes a printing plate having a positive-working heat-imagable coating, comprising a polymeric component, an insolubilizer compound and, preferably, an IR absorber. WO 99/08879 describes the use of such a coating in the manufacture of electronic parts, for example PCBs. WO 00/20222 describes improvements to that technology. To improve mechanical properties, such as resistance to flexure, scratching and cracking, the polymeric component comprises a defined vinylphenol alkylacrylate copolymer, providing at least 20% of the weight of the composition.
While the compositions of WO 00/20222 do have better mechanical properties for PCB applications than the prior compositions of WO 99/08879 they also suffer from a poorer exposed:unexposed dissolution ratio then the prior compositions. Thus a compromise normally must be reached between good imaging properties and good mechanical properties, with neither being optimized. Also, coating them onto substrates is not always easy. In particular, coating onto certain copper substrates is far from easy. Further, they have only moderate tolerance to film weight variations.
To our surprise we have found that our negative-working compositions currently used for lithographic plates may be modified to have the robustness needed for use in the manufacture of electronic parts, with maintenance of excellent imaging properties.
Furthermore, the modified compositions offer excellent coating characteristics.
A composition for use in the present invention is heat-sensitive in that localized heating of the composition, preferably by suitable radiation, initiates a decrease in the aqueous developer solubility of the exposed areas.
Therefore according to one aspect of the present invention there is provided a method of making an electronic part, the method comprising carrying out the following steps in the order given:
a) delivering heat imagewise to a precursor of the electronic part, the precursor having a surface carrying an image-forming layer including a polymeric substance, a cross-linking agent and a thermally-activated acid generator, wherein the polymeric substance comprises a polymer containing units derived from a vinylphenol and units derived from an alkyl acrylate;
b) heating the precursor overall; and
c) developing the precursor in an aqueous developer in order selectively to remove the image-forming layer in regions to which said heat was not delivered imagewise in step a).
Preferably the backbone carbon atoms of the polymer which carry respectively the phenolic and alkoxycarbonyl moieties are spaced by unsubstituted backbone carbon atoms (xe2x80x94CH2xe2x80x94).
Preferably the polymer is defined by the formula 
wherein R1 represents a hydrogen atom or alkyl group, R2 represents a hydrogen atom or alkyl group, R3 represents an alkyl or hydroxyalkyl group, and the ratio n/m is in the range 10/1 to 1/10.
We have found the method set out above, preferably using a polymer of formula I, to be a very effective one. The precursors we have used in such a method have excellent performance, with good imaging and development qualities and, at the same time, good mechanical properties, for example excellent flexibility, peel resistance, crack resistance and scratch resistance. Heretofore, despite considerable efforts, using other polymeric substances, we did not find it possible to optimise imaging and development qualities whilst maintaining the mechanical properties required of an electronic part precursor, in particular of an electronic part precursor of a thin flexible nature and/or which is coated on both sides. Furthermore the imaging and development qualities are maintained over an unusually wide range of coating thicknesses, typically from 2 to 10 gmxe2x88x922, and there appears to be no difficulty in achieving a good coating on copper substrates.
According to a second aspect of the invention there is provided a precursor for an electronic part, which precursor comprises a surface (not being an aluminum surface which has undergone electrograining, anodising and post-anodic treatments) and a layer carried by the surface, the layer being as defined above with reference to the first aspect.